The tissue distribution and clearance of radiolabeled microcystin-LR administered to Atlantic salmon via i.p. injection has been re-examined using uniformly C-14-labeled toxin, Significant differences were found to exist between these results and those obtained when fish received an i.p. injection of tritium-labeled dihydromicrocystin-LR. In addition, MeOH liver extracts were assayed by both phosphatase assay and C-14 counts and the results compared with the total levels of incorporation determined by digestion and subsequent C-14 counting of the same liver tissues. An attempt to investigate the metabolism and to document the putative products was also undertaken. It was found that microcystin-LR was extensively metabolized to compounds that are more polar than the parent compound. (C) 1997 Elsevier Science Ltd.